Conventionally, as a lighting apparatus that lights a high-intensity discharge lamp, such as a high-pressure mercury lamp, a high-pressure sodium lamp, and a metal halide lamp, a discharge lamp lighting apparatus shown in FIG. 5 is well known. Referring to FIG. 5, a discharge lamp lighting apparatus 100 comprises: an input power supply Vi that supplies a DC voltage to the discharge lamp lighting apparatus 100; a power supply circuit 1 that reduces an output voltage of the input power supply Vi and outputs the resultant voltage; a bridge circuit 4 that applies an output voltage Vo of the power supply circuit 1 to a discharge lamp (e.g., a high-intensity discharge lamp such as a metal halide lamp) 5 by switching the polarity of the output voltage Vo to perform AC operation of the discharge lamp 5; a lighting circuit 6 that starts the discharge lamp 5; a control circuit 102 that controls the power supply circuit 1; and a drive circuit 103 that drives the bridge circuit 4.
In the discharge lamp lighting apparatus 100, the power supply circuit 1 comprises: a switching element Q5; a diode D5; a choke coil L1; and a capacitor C1, and is a chopper circuit that drops the input voltage Vi to a predetermined voltage by ON\OFF operation of the switching element Q5 on the basis of an output signal from the control circuit 102. In this case, the control circuit 102 detects the output voltage Vo and tube current Io from the power supply circuit 1, and controls the feed-back of the power supply circuit 1 on the basis of the detected values so as to supply necessary power to the discharge lamp 5.
Further, the bridge circuit 4 is a full-bridge circuit that comprises four switching elements Q1, Q2, Q3, and Q4 and diodes D1, D2, D3, and D4 that are connected to the switching elements in parallel therewith. The bridge circuit 4 alternately performs ON/OFF operation of one pair of the switching elements Q1 and Q4 and the other pair of the switching elements Q2 and Q3 on the basis of an output signal from the drive circuit 103, thereby supplying power with AC low-frequency rectangular waves to the discharge lamp 5 so as to stably keep the lighting operation of the discharge lamp 5. Furthermore, the lighting circuit 6 is an igniter that generates high-voltage pulses for starting the discharge lamp 5. Although not shown, the lighting circuit 6 comprises a trigger circuit serving as a pulse generator and a pulse transformer that increases a voltage of the pulse.
In general, high-voltage pulses are applied to a discharge lamp (e.g., a high-intensity discharge lamp such as a high-pressure mercury lamp, a high-pressure sodium lamp, and a metal halide lamp) upon lighting the discharge lamp, the breakdown then occurs in the discharge lamp, and the discharge operation shifts from glow discharge to arc discharge. In this case, lamp impedance of the discharge lamp is dramatically decreased, charges stored in the capacitor C1 in the power supply circuit 1 are therefore rapidly discharged, and overcurrent is instantaneously generated and flows to the discharge lamp 5. A conventional discharge lamp lighting apparatus, such as the discharge lamp lighting apparatus 100 shown in FIG. 5, has a problem that the overcurrent causes the abrasion of an electrode in the discharge lamp and the lifetime of the discharge lamp thus becomes short. Therefore, as shown in FIGS. 6 and 7, a discharge lamp lighting apparatus that suppresses the overcurrent flowing to the discharge lamp is proposed.
A discharge lamp lighting apparatus 200 shown in FIG. 6 comprises an overcurrent control circuit 207 in addition to the similar structure to that of the discharge lamp lighting apparatus 100 shown in FIG. 5, the overcurrent control circuit 207 detects the tube current Io flowing to the discharge lamp 5, compares the detected current Io with a specific reference value, and controls a drive circuit 203 so as to perform chopper operation of the bridge circuit 4 together with normal low-frequency operation thereof only during a period for determining that the detected current Io is overcurrent. As a consequence, the overcurrent at the start of the discharge operation is suppressed (e.g., refer to Patent Document 1).
Further, a discharge lamp lighting apparatus 300 shown in FIG. 7 comprises a lighting detecting circuit 307 in addition to the similar structure to that of the discharge lamp lighting apparatus 100 shown in FIG. 5. The lighting detecting circuit 307 monitors the output voltage Vo of the power supply circuit 1, and detects the lighting operation of the discharge lamp 5. A drive circuit 303 sets a gate voltage or base current of the switching elements Q1 to Q4 of the bridge circuit 4 to be low before detecting the lighting detecting circuit 307 detects the lighting operation of the discharge lamp 5, thereby limiting the current flowing to the switching elements Q1 to Q4. After the lighting detecting circuit 307 detects that the discharge lamp 5 is lit, the gate voltage or base current is increased to a regular value. Thus, the overcurrent upon starting the discharge lamp is suppressed (e.g., refer to Patent Document 2).
Patent Document 1: Japanese Patent No. 3,258,758 (Claims 1 and 2 and FIG. 1)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-5185 (Claim 3 and FIG. 7)